1. Field of the Invention
The present invention generally relates to magnetic field sensor devices and more particularly relates to such devices that measure the amplitude of an incident magnetic field in a sensor plane.
2. Description of the Prior Art
Magnetometers and other magnetic sensing devices have many diverse applications including automobile detection, proximity sensors, magnetic disk memories, magnetic tape storage systems, magnetic strip readers, etc. Such devices typically provide one or more output signals that represent the magnetic field sensed by the magnetic sensing device.
Magnetic sensor devices typically include one or more sensor elements that are formed from a magnetoresistive material. The resistance of a magnetoresistive material typically changes when exposed to an incident magnetic field. Thus, to detect an incident magnetic field, most magnetic sensor devices simply sense the change in the resistance of the magnetoresistive material, and provide an output signal that indicates the presence of, or has an amplitude that is a function of, the incident magnetic field.
Common magnetoresistive materials include Anisotropic Magnetoresistive (AMR) materials, Giant Magnetoresistive (GMR) materials, and Colossal Magnetoresistive (CMR) materials. The resistance of AMR materials typically only changes a few percent change when exposed to an incident magnetic field. AMR materials are typically anisotropic with respect to the supplied current direction and incident field direction. Under limited conditions, however, AMR materials can be isotropic with respect to the incident field direction.
The resistance of GMR materials can change several hundred percent when exposed to an incident field. GMR materials are typically formed using multilayer films to produce a giant magnetoresistive effect. GMR materials are typically isotropic with respect to current direction, but can be anisotropic or isotropic with respect to the incident field direction depending on the type of crystal and shape structure. AMR and GMR materials are further discussed in U.S. Pat. No. 5,569,544 to Daughton.
CMR materials have the greatest magnetoresistive effect in response to an incident magnetic field. The resistance of a CMR material can change up to a 10.sup.6 percent. Most CMR materials are intrinsically isotropic in nature with respect to the supplied current and the incident magnetic field direction.
The response curve for a magnetoresistive material is often defined with the amplitude of the incident magnetic field along the X-axis, and the resulting resistance of the magnetoresistive material along the Y-axis. CMR and some GMR magnetoresistive materials can often have both a symmetrical and isotropic response curve. A symmetrical response curve is one that is symmetrical about the Y-Axis. That is, the magnetoresistive material satisfies the equation R(H)=R(-H), where H is the incident magnetic field.
For CMR and some GMR magnetoresistive materials, the response curve is not perfectly symmetrical because of hysteresis effects. FIG. 1 shows a response curve for a typical CMR magnetoresistive material. The response curve is nearly symmetrical (R(H).apprxeq.R(-H)) about H=0, with some hysteresis shown. For CMR and some GMR magnetoresistive materials, the hysteresis effects are small and can be effectively ignored except in the most sensitive magnetic sensor applications. As indicated above, the response curve of CMR and some GMR magnetoresistive materials is also isotropic. An isotropic response curve is one that is independent of the direction of the incident magnetic field, usually within a sensor plane.
FIG. 2 shows a schematic of a typical resistance measurement of a magnetoresistive film 20. Only a portion of the magnetoresistive film 20 is shown. Input current terminals 22 and 24 are electrically connected to the magnetoresistive film 20, as shown. A current source 26 is then connected between input current terminals 22 and 24 to provide a current through the magnetoresistive film 20. Output voltage terminals 28 and 30 are also electrically connected to the magnetoresistive film 20, as shown. A volt meter 32 measures the voltage generated between the output voltage terminals 28 and 30. The voltage measured by volt meter 32 is proportional to the resistance of the magnetoresistive film 20.
To measure the response curve of a magnetoresistive material, an incident magnetic field H 38 is provided to the film 20 at an angle .theta. 34 relative to a reference direction 36 in the sensor plane. As indicated above, CMR and some GMR magnetoresistive materials are typically isotropic, exhibiting the same response curve regardless of the direction .theta. 34 of the incident magnetic field in the sensor plane.
Because the amplitude of the incident magnetic field is important regardless of the direction in some applications, such as in an electromagnetic radiation monitor, it is often desirable to provide a magnetic field sensor that senses a magnetic field isotropically. In such a magnetic sensor device, the output signal is relatively independent of the direction of the incident magnetic field, at least within the sensor plane. Another advantage of sensing a magnetic field isotropically is that the sensitivity of the magnetic field sensor may be increased because all components of the magnetic field in a sensor plane are measured, resulting in a higher value of total field than a single component sensor would provide.
Despite the isotropic nature of CMR and some GMR materials, many prior art magnetic sensor devices introduce anisotropy into the response thereof. A primary limitation of many prior art magnetic sensor devices is that the shape of the magnetoresistive sensor element is rectangular or otherwise not symmetrical in the sensor plane. This typically introduces shape anisotropy into the response of the sensor. Shape anisotropy can significantly reduce the isotropic nature of the magnetic field sensor.
One approach to providing an isotropic magnetic field sensor is to use a proton precession magnetometer. A proton precession magnetometer uses the same principles as nuclear magnetic resonance (NMR). That is, the resonance frequency of a proton is dependent on the total incident magnetic field. While this approach may provide an isotropic magnetic field sensing function, the complexity and cost of such systems can be prohibitively large for many applications.